Core glass in the alkali-zinc-silicate glass system for an fiber-optic light guide and fiber-optic light guide made with said core glass

ABSTRACT

A core glass and a fiber-optic light guide made from it and a cladding glass are described. The core glass is in the alkali-zinc-silicate system and contains, in Mol % on an oxide basis: 54.5-65, SiO 2 ; 18.5-30, ZnO; 8-20, Σ alkali metal oxides; 0.5-3, La 2 O 3 ; 2-5, ZrO 2 ; 0.02-5, HfO 2 ; 2.02-5, Σ ZrO 2 +HfO 2 ; 0.4-6, BaO; 0-6, SrO; 0-2, MgO; 0-2, CaO; 0.4-6, Σ alkaline earth metal oxides; 0.5-3, Li 2 O, but no more Li 2 O than 25% of the Σ alkali metal oxides; &gt;58.5, Σ SiO 2 +ZrO 2 +HfO 2 . A molar ratio of Na 2 O/K 2 O is from 1/1.1 to 1/0.3. A molar ratio of ZnO to BaO is greater than 3.5.

CROSS-REFERENCE

The invention described and claimed herein below is also described inGerman Patent Application DE 10 2007 063 463.5-45, which was filed onDec. 20, 2007 in Germany. The aforesaid German Patent Applicationprovides the basis for a claim of priority of invention for theinvention claimed herein below under 35 U.S.C. 119 (a) to (d).

BACKGROUND OF THE INVENTION

1. The Field of the Invention

The invention relates to a core glass in the alkali-zinc-silicate glasssystem for a fiber-optic light guide and a fiber-optic light guide madefrom this core glass.

2. The Related Prior Art

Fiber-optic light guides are increasingly widely used for lighttransmission in the most different engineering and medicinal fields,e.g. in general industrial engineering, in lighting and trafficengineering, in the automobile industry, in dentistry, in endoscopy,etc. Fiber-optic light guides, which comprise individual fibersassembled into a fiber bundle, are usually made from glass because ofits good thermal and chemical resistance. The individual light fibersguide the light by total reflection. The most widely used light guidefibers are the step index fibers, which comprise a core made from a coreglass, which has a constant index of refraction over its cross-section.The core glass is surrounded by a cladding made of cladding glass, whichhas a lower index of refraction than the core glass. The totalreflection occurs at the boundary surface between the core glass and thecladding glass.

The amount of light, which can be coupled into this sort of fiber, isproportional to the square of the numerical aperture (NA) of the fiberand the cross-sectional area of the core fiber.

The attenuation of the light in the fiber also plays a great role aswell as the numerical aperture. Thus only a glass that has a lowattenuation of light can be used as a core glass. Because of the highpurity requirements the raw materials for the glass melt of this sort ofcore glass are very expensive, which can lead to a high cost for thissort of optical fiber and thus the manufactured light guide. Furthermoretoxic ingredients, such as PbO, CdO, BeO, Tl₂O, and ThO₂, can no longerbe used because of environmental considerations.

Besides the light flux, which the fiber-optic light guide transmits,frequently poor transmission of color shades of the light by the lightguide plays a significant role. Because of the spectral transmissiondependence of the core glass, which the fiber contains, a more or lessstrong shift in the color location of the coupled light source occurs,which is made most notable by a yellowish tinge or tint of the lightissuing from the light guide. This causes trouble in applications thatrequire a color neutral reproduction, e.g. in medicinal endoscopy, inphotographic image documentation to differentiate between healthy andmalignant tissue.

The reliability of the fiber, especially in mobile applications, i.e.which depends on its resistance to aging due to stress caused bytemperature changes between about −50° C. and 110° C., its resistance tomechanical stresses, especially resistance to vibration and chemicalresistance to environmental influences, is of importance. Especially thehydrolytic resistance and the acid resistance of the core glass are ofimportance. The density of the fibers is also of importance, since ithas a direct influence on the fuel consumption and load on the aircraftor automobile including the fibers. The density of the step index fiberis primarily determined by the density of the core glass.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an economically madecore glass for an optical step index fiber and an optical fiber withthis core glass.

It is another object of the present invention to provide core glass thatespecially only reacts with the cladding glass to a small degree orextent.

It is a further object of the present invention to provide a core glassand especially an optical fiber made from it, which withstands harshconditions, for example treatment in an autoclave, and is asnon-reactive as possible under these conditions, especially the coreglass should be of acid resistance glass 1.

According to the invention these objects and others, which will be mademore apparent herein after, are attained by a core glass in thealkali-zinc-silicate system for a step index fiber, which contains, inMol % on an oxide basis:

SiO₂ 54.5-65 ZnO 18.5-30 Σ alkali metal oxides   8-20 La₂O₃ 0.5-3 ZrO₂  2-5 HfO₂ 0.02-5  ZrO₂ + HfO₂ 2.02-5  BaO 0.4-6 SrO   0-6 MgO   0-2 CaO  0-2 Σ alkaline earth metal oxides 0.4-6 Li₂O 0.5-3, but not more than25% of the sum total of the amounts, in Mol % on an oxide basis, of thealkali metal oxides, Σ SiO₂ + ZrO₂ + HfO₂ >58.5;wherein a molar ratio of ZnO:BaO is preferably greater than 3.5:1.Preferably the alkali metal oxides include Na₂O and K₂O and a molarratio of Na₂O:K₂O is 1/1.1 to 1/0.3.

The glass contains SiO₂ as glass former. Glass formation in this systemis not possible when the concentration of SiO₂ is not greater than 50Mol %. However the chemical resistance of the glass is insufficient whenthe concentration is just greater than 50 Mol %. To obtain the best acidresistance of the class 1.x (x=1, 2, 3) which is required here, a SiO₂concentration of at least 54.5 Mol % is required, preferably at least 55Mol %. So that the melting temperatures—and especially the refiningtemperatures are not too high, the glass contains a maximum of 65 Mol %of SiO₂. For refining temperatures of less than 1450° C. the glasscontains at most 61 Mol % of SiO₂. The reduced refining temperature isdecisive so that the glass takes up as little platinum as possible inthe platinum refining chamber and thus does not attain a yellowish tint.Thus the concentration range for SiO₂ in the glass of the presentinvention is 54.5 to 65 Mol %, preferably 55 to 61 Mol %.

The glass can also contain up to 5 Mol % of B₂O₃ as a glass former. Thisingredient lowers the viscosity curve and thus the melting temperature,the refining temperature and the processing temperature. However atconcentrations higher than 5 Mol % it impairs the chemical resistance ofthe glass so that primarily only 0 to 1 Mol % of B₂O₃ are used under thelimiting condition that the effective SiO₂ concentration c(SiO₂)−c(B₂O₃) is >55 Mol %. If an especially great chemical stabilityis required, it is preferable to completely avoid B₂O₃ i.e. the glass isB₂O₃-free since the required viscosity properties can be obtained byincluding other glass ingredients. However it has been shown that B₂O₃has a positive effect on the solarization properties of the glass. Whengreat value is placed on the solarization properties while taking asomewhat poorer chemical stability into the bargain, the solarizationproperties of the glass can be definitely improved by including from 0.2to 2 Mol %, especially from 1 to 2 Mol %, and preferably from 1 to 1.5Mol % of B₂O₃.

The glass can also contain P₂O₅ as an additional glass former. Howeverit is advantageous to keep the P₂O₅ amount as small as possible. Ifcomplex phosphates MPO₃ are used in the glass batch, they provide largeamounts of Group-3d colored oxide impurities, especially Fe₂O₃, whichcan lead to undesirable discoloration of the resulting glass. Use offree P₂O₅ can lead to undesirable and strong exothermic batch reactions,which make the batch melting process difficult to handle and thusexpensive. Furthermore this component can lead to undesirablecrystallization products together with BaO, ZrO₂ and/or HfO₂. For thisreason the glass of the present invention is limited to containing from0 to 0.5 Mol % of P₂O₅. Preferably the glass of the present inventiondoes not contain any P₂O₅, i.e. it contains no P₂O₅.

Finally the glass of the present invention can contain GeO₂ in theamount of 0 to 5 Mol % as an additional glass former. GeO₂ has apositive effect on the index of refraction of the glass and thus thenumerical aperture of the fiber. It can contribute also to the loweringof the viscosity and reduces the crystallization of the silicate phase.Its high price is a consideration against using it in the glass of theinvention. One skilled in the art must balance these benefits anddisadvantages in each case to determine whether the use of GeO₂ issensible economically. Although it makes sense to include GeO₂ fortechnical reasons, in most cases it should be avoided for economicreasons, because of its current high cost.

For its manufacture in conventional vessels at moderate temperatures theglass needs network modifiers in addition to network formers. Alkalimetal oxides in amounts from 8 to 20 Mol %, preferably 11 to 16 Mol %,are used for that purpose. If the fraction of alkali metal oxides in theglass is greater than 20 Mol % the chemical resistance suffers and thethermal expansion coefficient is increased too much, which leads to anincreased tendency to crack. If the amount of alkali metal oxides isless than 8 Mol %, it is difficult to melt glass, the tendency tocrystallize is increased and the thermal expansion coefficient is lessthan 7×10⁻⁶ K⁻¹, whereby the pre-stressing of the fiber describedfurther herein below decreases and thus the resistance to mechanicalstresses decreases.

The principle alkali metal oxides that are used in the glass of theinvention are Li₂O, Na₂O, and K₂O. The use of Rb₂O and Cs₂O istechnically possible in amounts of about up to 6 Mol %, but is ofinterest only in special cases because of their considerably higherprice. The ratio three principle alkali metal oxides, Li₂O, Na₂O, andK₂O, to each other and to the alkali metal oxides in the cladding glassis of importance for the core glass application. During the fiberdrawing process inter-diffusion processes of the most mobile ions,primarily the alkali metal ions, occur. These processes are desired upto a certain extent, since they lead to a chemical combination of thecore and cladding glass and thus guarantee the stability of the fiber.If these processes are too strong or ions diffuse, which contribute tothe formation of a crystalline phase by enriching or impoverishing thecore glass or cladding glass, they become counter-productive. Thus ionconcentration gradients between the core and cladding glass should bepresent, but they should not be too large.

Generally it is most advantageous when Na₂O is the alkali metal oxidewith the largest molar concentration, especially in glasses with acomparatively low SiO₂ content and thus higher alkali metal ionmobility. In glasses with higher SiO₂ content the K₂O content can bealmost exactly the same size as the Na₂O content, but it should notexceed its nominal value.

The ratio of Na₂O to K₂O should thus be between 1:1.05 to 1:0.3.Especially this ratio of Na₂O to K₂O is preferably from 1:0.95 to 1:0.4.

Special attention must be given to the Li₂O content of the glass of thepresent invention. The Li⁺ ion is the most mobile component of thesystem and contributes to the formation of crystalline phases such asLiAlO₂ especially in the presence of Al₂O₃ and ZrO₂. ZrO₂ is, asdescribed further herein below, absolutely necessary for adjustment ofthe required optical properties. The addition of Al₂O₃ to the core glassbeyond amounts due to impurities in the raw materials of the glass batchmust be avoided. Since many cladding glasses contain Al₂O₃, it would bereally suitable to entirely avoid using Li₂O in core glass, in order tominimize the danger of crystallization at the boundary surface betweenthe core glass and the cladding glass. However there are actually anumber of core glasses, which contain Li₂O.

When no Li₂O is contained in the core glass, a cladding glass thatcontains Li₂O will introduce Li₂O into the core glass it surrounds bydiffusion, which can negatively affect the optical quality of thecore-cladding boundary surface.

In order to be able to combine a given core glass with differentcladding glasses and to be able to make a plurality of differentproducts, it has proven important to provide a Li₂O content in the coreglass. This content should be sufficiently high, in order to suppressthe negative affect caused by diffusion from a Li₂O-containing claddingglass, and sufficiently small in order to avoid crystallization effectsin Li₂O-free, Al₂O₃-containing cladding glass. As has been found byconsiderably effort, this object is attained by include from 0.5 to 3Mol %, preferably from 1.0 to 2.5 Mal %, and especially preferably from1.2 to 2.2 Mol % of Li₂O in the core glass according to the invention.Furthermore the Li₂O fraction of the total alkali metal oxide contentshould not exceed 25%, preferably 20%.

Furthermore the core glass contains ZnO in an amount of 18.5 to 30 Mol%, preferably from 20 to 25 Mol %. ZnO similarly acts as a networkmodifier and moreover serves to adjustment of the optical properties ofthe core fiber.

In contrast to the oxide ingredients described above ZnO acts as networkmodifier, which causes a more or less constant shift of the viscositycurve over all temperatures. ZnO causes a stronger decrease at highertemperatures and a weaker decrease to an increase at lower temperatures,i.e. a tilting of the viscosity curve. This is a very desirableproperty, since the viscosity during melting (T<1300° C.) and refining(T<1450° C.) should be low at as low as possible temperatures. Thisminimizes the reaction of the core glass with the vessel material andreduces the yellowish cast or tinting of the core glass, which is due torelease of platinum from the vessel material. In order to guarantee acompatibility with suitable cladding glasses during fiber drawing, thetemperature may be such that the viscosity is about 10^(6±1) dPa*s, butshould not be too low, i.e. the temperature should be above 900° C. atthe processing temperature V_(A) (viscosity 10⁴ dPa*s) and thetemperature should be above 680° C., preferably above 700° C., at thesoftening point E_(W) (viscosity 10^(7.6) dPa*s).

Preferably the ZnO has a high index of refraction increment and thus thecore glass has a high index of refraction and thus a high numericalaperture. Furthermore ZnO can be obtained economically in high purity(relation to color-causing impurities) because of its low cost. Ittransmits UV radiation comparatively far into the ultraviolet range (lowself-absorption), so that the visible spectral range is hardlyinfluenced at all by the presence of ZnO in the core glass. Thecombination of the high index of refraction increment, reduced selfabsorption and high purity make ZnO an ideal ingredient of thefiber-optic core glass, in which low attenuation is required. Anadditional positive characteristic of ZnO, especially in contrast toBaO, which is usually used in fiber-optic glass as the main agent for ahigh index of refraction, is the formation of only one singlecrystalline zinc-enriched phase in the R₂O—ZnO—SiO₂ system. Several ofthese troublesome phases exist in corresponding BaO(SrO) systems. Forthat reason the ZnO systems are preferred in contrast to the BaOsystems, since they are less inclined toward crystallization in thegreater concentration ranges, they are obtained in high purity and havea somewhat lower UV self-absorption. Fibers made with a core glasshaving a high ZnO content are characterized by a lower attenuation and asmaller tendency to crystallize than BaO based systems.

The present core glass can contain BaO, but only in minor amounts offrom 0.4 to 6 Mol %, preferably in amounts from 0.4 to 5 Mol %.

Similarly ZnO like BaO provides a high refractive index increment andcontributes to the dissolution of acidic ingredients, such as SiO₂ andZrO₂, because of its high basicity like the alkali metal oxides. HoweverBaO is more inclined to crystallize than ZnO and should be used only inthe minor or subordinate amounts. Likewise ZnO is preferred on thegrounds of its high purity and low self-absorption properties, asdescribed above.

BaO can be completely or partially replaced by SrO (0 to 6 Mol %,preferably up to 5 Mol %). However no substantial advantages areobtained by doing that. SrO is more expensive than BaO at the samepurity. Accordingly it is not used or only used to a minor extent. CaOand MgO (in amounts of 0 to 2 Mol % each) can similarly replace a partof the BaO, however they have a negative effect on index of refraction,acid resistance and impurity level, so that it is preferable to avoidthem.

In order to attain a core glass with the satisfactory properties, thesum of the alkaline earth metal oxides should not exceed 6 Mol % and thealkaline earth metal oxide content should not be too large in comparisonto the ZnO content. The molar ratio of ZnO:Σ alkaline earth metal oxidesshould be at least greater than 3.5:1, preferably >4:1.

The green glass system is defined by the above-mentioned ingredients. Toachieve a high index of refraction with satisfactory chemical resistanceand small crystallization tendency at the same time these ingredientsare not sufficient and further auxiliary oxide ingredients must becalled upon. Especially La₂O₃, ZrO₂ and HfO₂ should be mentioned asauxiliary oxide ingredients.

La₂O₃ is an ingredient with a high index of refraction increment and isused in a concentration range of 0.5 to 3.0, preferably 1.5 to 2.9, andespecially preferably 2.0 to 2.75 Mol %. If the amount of La₂O₃ is lessthan 0.5 Mol % the index of refraction increment is too low and thedesired index of refraction must be obtained by including more of otheroxide ingredients, which usually have other disadvantages. For thatreason the lower amounts of 0.5 to 1.5 Mol % are only used in specialcircumstances. With amounts above 2.75 Mol % the crystallizationincreases too much so that the glass can no longer be used for all fibertypes, e.g. small fiber diameters, which can no longer be produced withgood yield because of long idle time during the drawing process. Forspecial cases these amounts are still significant. Above 3 Mol % thecrystallization can be scarcely controlled, so that the use of a glasscontaining these amounts of La₂O₃ for fiber applications must beavoided. Furthermore La₂O₃ is very reactive toward SiO₂-containingfire-resistant materials. For these reasons the use of the maximum 3 Mol% of La₂O₃ remains limited, in order to be able to manufacture withstandard melt vessels.

La₂O₃ can be replaced by equivalent amounts of Gd₂O₃ and Lu₂O₃ accordingto the art. However since these oxides are expensive and not very pure,this is usually avoided. Y₂O₃ has a smaller index of refractionincrement and a higher cost and thus is not used. In this case the useof small amounts to suppress crystallization appears to be ineffectiveso that as a result there is no reason to use Y₂O₃. The same is true forSc₂O₃.

ZrO₂ and HfO₂ are two essential ingredients for producing a high indexof refraction and good chemical resistance. A minimum amount of 0.02 Mol% of HfO₂ is required, since HfO₂ counteracts the tendency of the glassto crystallize. A minimum content of 2.02 Mol % for the sum of theamounts of ZrO₂ and HfO₂ is required to obtain a high index ofrefraction. However if the maximum amount of the sum of these oxidesexceeds 5 Mol %, the meltability decreases and the tendency tocrystallize increases. Considering these limitations regarding the sumof these two oxides the glass should contain from 2 to 5 Mol % of ZrO₂and from 0.02 to 5 Mol % of HfO₂, preferably from 2 to 4.08 Mol % ofZrO₂ and from 0.02 to 4.1 Mol % HfO₂. Thus it is surprising that apositive effect on the tendency to crystallize starts to appear in spiteof the small content of HfO₂. Price and purity of HfO₂ are generallycurrently still not competitive with the price and purity of ZrO₂, sothat HfO₂ is seldom used in amounts over 1 Mol % for purely pragmaticreasons. Generally the HfO₂ content is less than 0.3 Mol %.

Furthermore it has been shown that a minimum amount of the sum ofSiO₂+ZrO₂+HfO₂ is required to obtain a good acid resistance. The sum ofSiO₂+ZrO₂+HfO₂ should be at least 58.8 Mol %, preferably at least 59 Mol%.

The best acid classes 1.x are only obtained in exceptional cases whenthe sum of SiO₂+ZrO₂+HfO₂ is below 58.8 Mol %. In the range between 58.8Mol % and 59 Mol % acid class 1 predominates with a few cases of acidclass 2 and in the range greater than 59 Mol % glass of acid class 1 isproduced. However even in the range greater than 59 Mol % but near 59Mol % glass of acid glass 1.0 is still not produced, but instead thesubclasses 1.1, 1.2, and 1.3 result. Although these acid classes exhibitno removal from the glass surface, they still have optical variations ofthe glass surface (interference colors).

Further the glass contains 0-3 Mol % Nb₂O₅ and Ta₂O₅. Both oxides can beused in a similar manner for adjustment of the optical properties(n_(d)-υ_(d)). However both oxides, especially Ta₂O₅, are more expensivethan the other high index-of-refraction oxides (ZrO₂, La₂O₃, HfO₂), sothat whether it is better to obtain the n_(d) by using the other oxidesshould be carefully considered. Especially in the case of Nb₂O₅ itshould be considered that the current purity of Nb₂O₅ is notsatisfactory and also boundary surface effects increasing damping occurwith some of the cladding glasses. For these reasons the glasspreferably contains no Nb₂O₅.

In cases in which the refining is not free of refining agents, arefining with As₂O₃ and/or Sb₂O₃ is preferred in amounts of up to 0.5Mol %. Preferably As₂O₃ is used, since it provides better attenuationresults than Sb₂O₃ at the blue edge. However it is has been shown thatglass refined with Sb₂O₃ has better solarization properties than a glassrefined with AS₂O₃. If there is special value associated withsolarization resistance of the glass, then it is preferable to refineusing Sb₂O₃ and to keep the AS₂O₃ as small as possible. The glass rawmaterial should then be as free of As₂O₃ as possible. The As₂O₃ contentshould not exceed 500 ppm, preferably 100 ppm, especially 50 ppm (Molarbasis). An amount of 0.05 to 0.4 Mol %, especially 0.1 to 0.2 Mol %,Sb₂O₃ is preferred to increase solarization resistance. Sulfates andchlorides should be avoided as refining agents, since the attenuation issomewhat negatively effected.

The content of colored impurities, such as Fe₂O₃, Cr₂O₃, NiO, CuO,colored d-elements and rare earth oxides should be as low as possible.An upper limit for the sum of these colored impurities of <10 ppm,preferably <1 ppm, should not be exceeded.

Furthermore it has been shown that the solarization stability of theglass can be further improved by addition of small amounts of WO₃, Bi₂O₃and/or TiO₂. Generally these oxides exert a negative influence on theattenuation. When these oxides are added to improve solarization, Bi₂O₃and TiO₂ should each only be present in the glass in amounts of from0.05 to 0.5 Mol %. On account of its high coloring power WO₃ should onlybe used in amounts of 0.05 to 0.35 Mol %. The preferred amount of eachof these oxides is from 0.1 to 0.3 Mol %. The oxides Bi₂O₃, WO₃ and TiO₂can each be used alone but they can also use in combination with eachother. A combination of one or more of these oxides with 0.2 to 2 Mol %of B₂O₃ exerts an especially positive influence on the solarizationstability. The combination of 1 Mol % B₂O₃ with 0.3 Mol % TiO₂ hasproven to be especially good.

Besides the requirement for small attenuation a requirement for asneutral as possible a color temperature exists. The longer fibers madefrom these glasses frequently have a yellow tint because of Fe, Cr, andNi impurities in the glass batch used to make the core glass as well asPt impurities occurring because of the melting process. For this reasonit can be important to add small amounts of CoO to the glass so that itis a neutral color glass. The amounts of CoO to be used are 3 to 100ppb, preferably 5 to 50 ppb. The color temperatures increase to valuesup to over 5700 K (Standard light D65, 3.8 m length).

EXAMPLE

To make the core glass a batch of commercially obtained raw materialswere placed in a platinum heated crucible with a silica glass crucibleinsert heated inductively at 1420° C. for a time interval of about 5 h.The outer temperature of the platinum crucible was measured with athermocouple. Heat is lost through the silica glass crucible so that thepyrometrically measured glass temperature can up to 80° C. less thanthat of the melt phase. After that a standing stage of about 0.75 hoccurs, during which the final melt remnants dissolve and refiningoccurs. After that a stirring stage occurs for about 15 min for grosshomogenization (silica glass stirrer). A fine refining is performed at1460° C. for 15 min. After that the glass melt is stirred at 1320° C.and cast or molded to form the desired glass product. This glass productis inserted in a laboratory cooling oven and cooled at a rate of 10 K/hto room temperature.

The results are summarized or tabulated in Table I. The glasscompositions are given in Mol % on an oxide basis. The symbols for theproperties have the following meanings: n_(d) represents the index ofrefraction; υ_(d) represents the Abbe number; α represents the linearthermal expansion coefficient in a temperature range between 20° C. and300° C. according to ISO 7991; Tg represents the glass transformationtemperature according to ISO 7884; ρ represents the density measuredaccording to the buoyancy flotation method according to Archimedesprinciple; Ew represents the glass softening temperature (glassviscosity of 10^(7.6) dPa*s), V_(A) represents the processingtemperature (glass viscosity of 10⁴ dPa*s); and CR is the climateresistance class according to ISO/WD 13384.

FR represents the stain or spot resistance. A polished plate is broughtinto contact with a standard acetate buffer solution with a pH of 4.6 at25° C. to measure the stain resistance. A stain is detected by itsblue-brown interference color (which corresponds to a layer thicknessfor the stain of 0.1 μm). The stain resistance is classified as follows:

Class 0: no stain after 100 hours of treatment,

Class 1: a stain observed after 100 hours of treatment,

Class 2: a stain observed after 6 hours of treatment, and

Class 3: a stain observed after 1 hour of treatment.

SR represents the acid resistance class according to ISO 8424.

AR represents the alkali resistance class according to ISO 9689.

OEG (T) represents the upper devitrification temperature in ° C.

OEG (Ig η) represents the common logarithm of the viscosity at the upperdevitrification temperature.

TABLE I EXEMPLARY CORE GLASS COMPOSITIONS, in Mol % Example No: Oxide 12 3 4 5 6 7 SiO₂ 59.73 59.32 59.67 59.68 58.51 58.96 56.77 Li₂O 2.011.99 1.99 1.99 2.04 1.95 1.97 Na₂O 7.76 7.72 7.75 7.82 7.98 8.13 7.95K₂O 4.83 5.16 5.14 5.08 5.04 5.19 4.94 BaO 0.78 0.42 0.48 0.43 0.42 0.490.42 ZnO 20.74 21.04 20.72 20.73 20.56 20.82 24.36 La₂O₃ 2.10 2.12 2.122.12 2.12 0.54 1.24 ZrO₂ 1.93 2.11 2.11 2.12 2.13 3.88 2.22 HfO₂ 0.020.02 0.02 0.02 0.02 0.04 0.02 As₂O₃ 0.12 0.12 — 0.04 0.12 0.04 0.12ΣSiO₂ + ZrO₂ + HfO₂ 61.68 61.45 61.80 61.82 60.66 62.88 59.01 B₂O₃ — — —— 1.08 — — Al₂O₃ — — — — — — — Nb₂O₅ — — — — — — — Ta₂O₅ — — — — — — —Sb₂O₃ — — 0.04 — — — — n_(d) 1.58617 1.58698 1.58647 1.58628 1.586331.58216 1.58825 ν_(d) 51.87 51.68 51.62 51.83 52.37 51.27 51.06 α [10⁻⁶K⁻¹] 8.78 8.82 8.83 — 8.82 8.62 8.74 ρ [g/cm³] 3.1205 3.1209 3.1174 — —3.0439 3.1330 Tg 536 547 539 — 527 546 534 E_(w) 704 — — — 693 ▪ — V_(A)947 — — — 933 974 931 CR 1 1 1 — — 1 1 FR 0 1 1 — 0 0 0 SR 1.0 1.0 1.0 —1.0 1.0 1.2 AR 1.0 1.0 1.0 — 1.0 1.0 1.0 OEG (T) 1055 — 1045 — 1005 11001025 OEG (lg η) 3.10 — — — 3.37 2.99 3.18 Example No: Oxide 8 9 10 11 1213 14 15 SiO₂ 55.45 58.47 54.65 55.60 56.25 55.62 55.39 55.89 Li₂O 0.971.96 2.09 2.04 1.88 1.75 1.98 1.93 Na₂O 6.99 7.25 7.68 7.63 7.39 6.976.58 6.62 K₂O 3.08 2.98 3.22 3.06 3.01 3.07 3.05 3.09 BaO 5.77 4.5 4.874.75 4.74 4.71 4.70 4.66 ZnO 21.56 19.22 21.39 20.97 20.57 21.36 21.2721.65 La₂O₃ 2.67 2.46 2.67 2.63 2.59 2.68 2.62 2.64 ZrO₂ 3.40 0.03 0.031.61 3.14 3.67 3.27 3.40 HfO₂ 0.04 3.01 3.28 1.60 0.23 0.05 0.04 0.04As₂O₃ 0.12 0.12 0.12 0.12 0.12 0.12 0.12 0.12 ΣSiO₂ + ZrO₂ + HfO₂ 58.8961.51 57.96 58.81 59.62 59.34 58.69 59.24 B₂O₃ — — — — — — 1.00 — Al₂O₃— — — — — — — — Nb₂O₅ — — — — — — — — Ta₂O₅ — — — — — — — — Sb₂O₃ — — —— — — — — n_(d) 1.62468 1.60984 1.62384 1.62265 1.62361 1.62353 1.619291.62122 ν_(d) 49.92 51.68 50.44 50.26 49.94 49.94 — 50.14 α [10⁻⁶ K⁻¹]8.17 7.99 8.38 8.33 8.22 8.14 8.12 8.14 ρ [g/cm³] 3.4681 3.4598 3.57873.4496 3.4392 3.4293 3.378 3.3992 Tg 588 566 566 570 563 565 555 567E_(w) 738 738 723 724 726 730 716 725 V_(A) 955 977 942 945 946 950 936947 CR — 1 — — — 1 1 1 FR — 0 — — — 0 0 0 SR — 1.0 — — — 1.0 1.0 1.0 AR— 1.0 — — — 1.0 1.0 1.0 OEG (T) 1125 965 1045 1050 1145 1180 1005 1075OEG (lg η) 2.5 4.12 3.03 3.01 2.34 2.12 3.32 2.86

The manufacture of optical step index fibers from a multicomponent glassoccurs either by the so-called double crucible method or the rod-tubemethod. In the case of both methods the core glass and the claddingglass are heated to temperatures, which correspond to a viscosity rangebetween 10⁴ and 10³, and are drawn to form a fiber. So that a stablefiber with low attenuation can be produced, the core and cladding glassmust have properties, such as viscosity behavior, thermal expansion,crystallization tendency, among others, such that they are compatiblewith each other. Particularly reactions, such as diffusion andcrystallization, cannot occur between the core glass and the claddingglass at the boundary surfaces between the core and the cladding, whichinterfere with a total reflection of the light in the fiber core andincrease the attenuation. Moreover the mechanical strength of the fibermust not be negatively affected by crystallization.

The cladding glass comprises a silicate glass, which has an index ofrefraction, which is at least 2% less than that of the core glass and aviscosity, which is preferably equal to or higher than the viscosity ofthe core glass, at the temperature at which the fiber is drawn. Thehigher viscosity of the cladding glass in comparison to that of the coreglass improves the stability of the drawing process. Furthermore thelinear thermal expansion coefficient α of the cladding glass should beequal to or especially at least about 2×10⁻⁶ K⁻¹ smaller than that ofthe core glass. The fiber cladding experiences a pre-stressing, whichincreases the mechanical stability of the fiber, because of its smallerthermal expansion coefficient. However the pre-stressing may not be solarge that problems occur with the fiber production. That is usuallyavoided when the difference between the thermal expansion coefficientsis under about 5-6×10⁻⁶ K⁻¹.

The index of refraction of the cladding glass n_(d) should be less than1.5, so that the fiber has a large numerical aperture NA. Glass with ahigher index of refraction should only be used for the cladding glass ifa smaller aperture NA is desired.

Furthermore the cladding glass preferably melts at a higher temperaturethan the core glass so that the fiber can be satisfactorily drawn andprocessed, as experiments have shown. This is especially true when thefibers are bundled to make a light guide and melted under radialpressure.

For economic reasons the known and available types of tubular glass areused for the cladding glass and the types of core glass are adjusted tofit the cladding glass, so that no crystallization or otherdevitrification effects occur at the core-cladding boundary surface.

The relevant tubular glasses may be divided into three groups, to whichdifferent representatives belong, according to their index ofrefraction, viscosity position, thermal expansion properties andavailability.

Glasses of group 1 include the so-called neutral glasses, such as thoseused as primary packaging agents in the pharmaceutical industry (e.g.SCHOTT Glass 8800 or 8412). These Li₂O-free borosilicate glasses arepreferred as cladding glasses for the above-described step index fibers.They are especially well suited as the core glass because their thermalexpansion coefficient is in a range of about 5 ppm/K. The low index ofrefraction value of about 1.49 permits a high aperture angle andaperture NA of the fiber.

Glasses of group 2 include Li₂O-containing borosilicate glass with ahigh B₂O₃ content over 15%. KOVAR® glasses (e.g. SCHOTT 8242, 8245, and8250) and UV transmitting borosilicate glasses (SCHOTT 8337b) fallwithin this group.

Glasses of group 3 include borosilicate glasses of the standard DIN ISO3585 (“Borosilicate glass 3.3”). However adjustment of these glasses isnot optimal because of their low expansion coefficients of 3 to 4×10⁻⁶K⁻¹. Also the mechanical strength of the fiber is less than that of thecladding glasses of group 2, since crystallization occurs at theboundary surface between core and cladding glass.

The cladding glass compositions of the different cladding glass groupsand their properties are tabulated or summarized in Table II. Thecladding glass compositions are given in weight percent on an oxidebasis.

TABLE II CLADDING GLASS COMPOSITIONS in Weight % on an oxide basis Group1 Group 2 Group 3 Range Ex. 1a Ex. 1b Range Ex. 2a Ex. 2b Ex. 2c RangeEx. 3a Oxide SiO₂ 70-78 75.1 73.9 62-70 68.0 68.5 67.1 75-85 80.6 Al₂O₃ 5-10 5.2 6.6  0-10 5.6 2.7 5.0 1-5 2.4 B₂O₃  5-14 10.5 9.6 >15 18.519.0 20.8 10-14 12.7 Li₂O — — — >0.1 0.5 0.7 0.7 — — Na₂O  0-10 7 6.6 0-10 6.9 0.7 2.5 2-8 3.5 K₂O  0-10 0.2 2.6  0-10 — 7.6 1.6 0-1 0.6 MgO0-1 — 0.01 0-5 — — — — — CaO 0-2 1.5 0.7 0-5 — — 0.6 — — SrO 0-1 — — 0-5— — — — — BaO 0-3 0.6 0.04 0-5 — 1.3 — — ZnO — — — 0-5 1.0 0.6 — — — F0-1 0.2 — 0-1 — — 0.8 — — Physical and Chemical Properties n_(d) — 1.4921.488 — 1.488 1.487 1.476 — 1.473 α [10⁻⁶ K⁻¹] — 4.9 5.5 — 5.1 5.0 4.2 —3.3 ρ [g/cm³] — 2.34 2.34 — 2.31 2.28 2.21 — 2.23 W — 1 1 — 3 3 3 — 1 S— 1 1 — 4 4 4 — 1 L — 2 2 — 3 3 3 — 2 The following is a list of thesymbols in Table II and their meaning: ‘Ex.’ represents ‘Example’ αdenotes the thermal expansion coefficient in a temperature range of 20°C. to 300° C. ρ denotes the density n [g/cm³] W denotes the hydrolyticresistance according to DIN ISO 720 S denotes the acid resistanceaccording to DIN 12116 L denotes the alkali resistance according to DINISO 695.

Optical fibers with diameters of 30, 50, and 70 μm were made fromselected core glasses from Table I, which were combined with selectedcladding glasses from Table II. The physical properties of these opticalfibers were measured. The optical fibers were made according to a knownrod-tube drawing method with a conventional rod-tube drawing machinewith a cylindrical oven according to the state of the art. Fiber opticlight guides were made from the individual optical fibers. The opticalfibers were glued together in a fiber bundle of a 2-3 mm diameter in anend sleeve and the optical end surfaces were prepared for input andoutput of light by grinding and polishing.

The attenuation of individual fibers of diameter 70 μm was measuredaccording to DIN 58141, Part 1 (cut-back method). Furthermore theaverage loop breaking diameter was measured, which reflects the basicstrength of the fiber. The loop breaking diameter is measured, when thefiber is arranged in a loop, whose diameter is slowly reduced until itreaches the breaking point, which corresponds to the loop breakingdiameter.

The spectral transmissions of test samples of the light guide withsample lengths of 1 m and 3.8 mm were measured according to the methoddescribed in DIN 58141, Part 2. The numerical aperture was also measuredfor a sample of length 1 m at 550 nm according to the method describedin DIN 58141, Part 3.

The color temperature of the light issuing from the fiber after passingthrough a certain fiber length was measured in relation to a standardlight source D65 (color temperature 6504 K) as a measure of the yellowtint or yellowness of the fiber. The more the color temperature deviatesfrom the color temperature of the standard light source D65, the greateris the yellow cast or tint.

The results are tabulated or summarized in Table III, “OPTICAL FIBERPROPERTIES I-NA RANGE about 0.54” and in Table IV “OPTICAL FIBERPROPERTIES II-NA-RANGE about 0.64”. Comparative data for lead-containingtypes of optical fiber with comparable numerical apertures of 0.54 andof 0.64 are given for comparison. The comparative data includes data fora platinum-free lead-containing glass that was produced without anattenuation increasing Pt influence and a platinum-containing glass,which was produced in a platinum-containing melt vessel.

It is apparent from the examples in these tables of the lead-free fibersnos. F3 to F5 with an NA-range of 0.54 that the lead-free fibersaccording to the present invention are equally as good as the currentmulti-usage fibers F1 and F2 of the prior art in regard to attenuation,transmission, and aperture angle, both in the case of the platinum-freeglass and the platinum-containing glass. The optical properties wereobtained for the lead-free core glass according to example 4 of table Iwith both a cladding glass of group I (F3) and also a cladding glass ofgroup II (F5). However it has been shown that optimum fibers cannot beproduced in every case by combining this new core glass of example 4according to the invention with a respective cladding glass of group 2.For example, the optical fiber made by combining the core glass ofexample 4 with the cladding glass 2c according to table 11 does not haveoptimum optical properties.

The examples F9 and F10 of the lead-free optical fiber with an NA-rangeof 0.64 have similar optical properties as the lead-containing opticalfibers F7 and F8. If e.g. the quality of the glasses F10 and F7 from theplatinum-containing melt vessel are compared, it is apparent that thelead-free optical fiber F10 has similar transmission values and apertureangle as the lead-containing optical fiber F7. The color temperature ofthe lead-free optical fiber sample with a 3.8 m length, namely 5501 K,is about 50 K below that of the lead-containing optical fiber, namely5546 K. The color temperature can be definitely increased by a smallamount of cobalt dopant, which increases the attenuation at about 500nm, as can be seen from example F12. The transmission drop to beexpected in this case is accepted because of a smaller color tint.

From example F12 it is also apparent that the high index of refractioncore glasses according to the invention are compatible with claddingglasses of group 2. Example F11 shows that the color temperature D65 canbe increased even over the optimum value of 6504 K by an appropriatelylarge amount of Co dopant.

TABLE III OPTICAL FIBER PROPERTIES I - NA-RANGE about 0.54 (Fiberdiameter 70 μm) Fiber Type No: F1 F2 F3 F4 F5 F6 Category Lead- Lead-Lead-free Lead-free Lead-free Lead-free containing containing fiber withfiber, PT- fiber with fiber with fiber fiber, Pt- Group 1 free, withGroup 2 Group 2 free quality Cladding Group 1 Cladding Cladding glassCladding glass glass, glass Counter example Core glass from Tab. I LeadLead 4 4 4 4 Example No: silicate silicate n_(d) = 1.582 n_(d) = 1.582Core glass from Tab. II 2b 2b 1b 1b 2b 2c Example No.: Theoretical NA0.54 0.54 0.54 0.54 0.55 0.58 NA, measured at 550 0.55 0.53 — 0.54 0.52— nm fur 1 m length Fiber attenuation, 70 μm . . . at 400 nm [dB/km] 607328 423 290 298 1937 . . . at 450 nm [dB/km] 423 244 351 230 280 2079 .. . at 550 nm [dB/km] 205 128 131 124 118 1571 . . . at 650 nm [dB/km]259 142 147 122 143 1280 Transmission, 70 μm, length 1 m . . . at 458 nm[%] 55 59.7 62.3 59.4 58.2 — . . . at 553 nm [%] 58 62.0 65.8 61.1 60.6— . . . at 654 nm [%] 58 61.9 65.8 61 60.6 — Color temperature D65 62306355 6205 6491 6357 — fur sample length 1 m [K] Breaking loop diameter,1.9 1.8 2.0 2.2 1.7 2.2 70 μm [mm]

TABLE IV OPTICAL FIBER PROPERTIES II - NA-RANGE about 0.64 (Fiberdiameter 70 μm) Fiber type No: F7 F8 F9 F10 F11 F12 Category Lead- Lead-Lead-free Lead-free Lead-free Lead-free containing containing fiber, Pt-fiber with fiber with fiber with fiber with fiber, Pt- free Pt- Pt-group 2 Pt- free Influence Influence cladding Influence, quality, NA andco- glass, Pt- NA 0.64 0.64 doping free Core glass from Tab. I Lead Lead15 15 15 15 Example No: silicate silicate n_(d) = 1.620 n_(d) = 1.620Core glass from Tab. II Group 2 Group 2 1b 1b 1b 2b Example No.:Theoretical NA 0.64 0.64 0.64 0.64 0.64 0.64 NA, measured at 550 0.640.61 0.64 0.63 — 0.63 nm fur 1 m length Fiber attenuation, 70 μm . . .at 400 nm [dB/km] 944 724 420 714 690 468 . . . at 450 nm [dB/km] 595372 316 566 600 366 . . . at 550 nm [dB/km] 237 169 144 213 519 198 . .. at 650 nm [dB/km] 236 140 133 242 712 185 Transmission, 70 μm, length1 m . . . at 458 nm [%] 60.7 59 58.9 60.7 — 59.1 . . . at 553 nm [%]64.9 62.0 61.6 66.2 — 61.8 . . . at 654 nm [%] 65.3 63.0 61.8 65.9 —61.9 Color temperature D65 5546 5714 5855 5501 6797 5854 fur samplelength 1 m [K] Breaking loop diameter, 2.1 2.0 1.7 1.9 — 1.7 70 μm [mm]

While the invention has been illustrated and described as embodied in acore glass in the alkali-zinc-silicate glass system for a fiber-opticlight guide and a fiber-optic light guide made from this core glass, itis not intended to be limited to the details shown, since variousmodifications and changes may be made without departing in any way fromthe spirit of the present invention.

Without further analysis, the foregoing will so fully reveal the gist ofthe present invention that others can, by applying current knowledge,readily adapt it for various applications without omitting featuresthat, from the standpoint of prior art, fairly constitute essentialcharacteristics of the generic or specific aspects of this invention.

What is claimed is new and is set forth in the following appendedclaims.

1. A core glass in the alkali-zinc-silicate system for a step indexfiber, which contains, in Mol % on an oxide basis: SiO₂ 54.5-65 ZnO18.5-30 Σ alkali metal oxides   8-20 La₂O₃ 0.5-3 ZrO₂   2-5 HfO₂ 0.02-5 Σ ZrO₂ + HfO₂ 2.02-5  TiO₂   0-0.5 BaO 0.4-6 SrO   0-6 MgO   0-2 CaO  0-2 Σ alkaline earth metal oxides 0.4-6 Li₂O 0.5-3 Σ SiO₂ + ZrO₂ +HfO₂ >58.5;

wherein a molar ratio of ZnO:Σ alkaline earth metal oxides >3.5:1, butwith the proviso that the core glass contains said Li₂O in an amountthat is not more than 25% of a sum total amount, in Mol % on said oxidebasis, of said alkali metal oxides present in the core glass.
 2. Thecore glass as defined in claim 1, containing from 2 to 4.08 Mol % ofsaid ZrO₂, from 0.02 to 3.3 Mol % of said HfO₂, and from 2.02 to 4.1 Mol% of said Σ ZrO₂+HfO₂.
 3. A core glass in the alkali-zinc-silicatesystem for a step index fiber, as defined in claim 1 and which contains,in Mol % on an oxide basis: SiO₂ 55-61 ZnO 20-25 Σ alkali metal oxides11-16 La₂O₃ 1.5-2.9 ZrO₂   2-4.08 HfO₂ 0.02-3.3  Σ ZrO₂ + HfO₂ 2.02-4.1 TiO₂  0-0.5 BaO 0.4-5   SrO 0.4-5   Σ alkaline earth metal oxides0.4-5   Li₂O 1-2.5 Σ SiO₂ + ZrO₂ + HfO₂ >59.


4. The core glass as defined in claim 1, wherein said alkali metaloxides comprise Na₂O and K₂O and a molar ratio of said Na₂O to said K₂Ois from 1:1.1 to 1:0.3.
 5. The core glass as defined in claim 1, whereinsaid alkali metal oxides comprise Na₂O and K₂O and a molar ratio of saidNa₂O to said K₂O is from 1:0.95 to 1:0.4.
 6. The core glass as definedin claim 1, wherein said molar ratio of said ZnO:said Σ alkaline earthmetal oxides >4:1.
 7. The core glass as defined in claim 1, containingfrom 0 to 5 Mol % of B₂O₃ and/or from 0 to 0.5 Mol % of P₂O₅, from 0 to5 Mol % of GeO₂ and/or from 0 to 6 Mol % of Rb₂O and/or from 0 to 6 Mol% of Cs₂O.
 8. The core glass as defined in claim 1, containing from 0.2to 2 Mol % of said B₂O₃.
 9. The core glass as defined in claim 1,containing from 0.05 to 0.4 Mol % of Sb₂O₃.
 10. The core glass asdefined in claim 9, containing from 0.1 to 0.2 Mol % of said Sb₂O₃. 11.The core glass as defined in claim 1, containing from 0.05 to 0.5 Mol %of Bi₂O₃ and/or from 0.05 to 0.5 Mol % of TiO₂ and/or from 0.05 to 0.35Mol % of WO₃.
 12. The core glass as defined in claim 11, containing from0.1 to 0.3 Mol % of said Bi₂O₃ and/or from 0.1 to 0.4 Mol % of said TiO₂and/or from 0.1 to 0.3 Mol % of said WO₃.
 13. The core glass as definedin claim 3, wherein said alkali metal oxides comprise Na₂O and K₂O and amolar ratio of said Na₂O to said K₂O is from 1:1.1 to 1:0.3.
 14. Thecore glass as defined in claim 3, wherein said alkali metal oxidescomprise Na₂O and K₂O and a molar ratio of said Na₂O to said K₂O is from1:0.95 to 1:0.4.
 15. The core glass as defined in claim 3, wherein saidmolar ratio of said ZnO:said Σ alkaline earth metal oxides >4:1.
 16. Thecore glass as defined in claim 3, containing from 0 to 5 Mol % of B₂O₃and/or from 0 to 0.5 Mol % of P₂O₅, from 0 to 5 Mol % of GeO₂ and/orfrom 0 to 6 Mol % of Rb₂O and/or from 0 to 6 Mol % of Cs₂O.
 17. The coreglass as defined in claim 3, containing from 0.2 to 2 Mol % of saidB₂O₃.
 18. The core glass as defined in claim 3, containing from 0.05 to0.4 Mol % of Sb₂O₃.
 19. The core glass as defined in claim 18,containing from 0.1 to 0.2 Mol % of said Sb₂O₃.
 20. The core glass asdefined in claim 3, containing from 0.05 to 0.5 Mol % of Bi₂O₃ and/orfrom 0.05 to 0.5 Mol % of TiO₂ and/or from 0.05 to 0.35 Mol % of WO₃.21. The core glass as defined in claim 20, containing from 0.1 to 0.3Mol % of said Bi₂O₃ and/or from 0.1 to 0.4 Mol % of said TiO₂ and/orfrom 0.1 to 0.3 Mol % of said WO₃.
 22. A step index fiber comprising acore and a cladding, wherein the core comprises a core glass and saidcore glass contains, in Mol % on an oxide basis: SiO₂ 54.5-65 ZnO18.5-30 Σ alkali metal oxides   8-20 La₂O₃ 0.5-3 ZrO₂   2-5 HfO₂ 0.02-5 Σ ZrO₂ + HfO₂ 2.02-5  TiO₂   0-0.5 BaO 0.4-6 SrO   0-6 MgO   0-2 CaO  0-2 Σ alkaline earth metal oxides 0.4-6 Li₂O 0.5-3 Σ SiO₂ + ZrO₂ +HfO₂ >58.5;

wherein a molar ratio of ZnO:Σ alkaline earth metal oxides >3.5:1, butwith the proviso that said core glass contains said Li₂O in an amountthat is not more than 25% of a sum total amount, in Mol % on said oxidebasis, of said alkali metal oxides present in said core glass.
 23. Astep index fiber comprising a core and a cladding, wherein the corecomprises a core glass and said core glass contains, in Mol % on anoxide basis: SiO₂ 54.5-65 ZnO 18.5-30 Σ alkali metal oxides   8-20 La₂O₃ 0.5-3 ZrO₂   2-5 HfO₂ 0.02-5 Σ ZrO₂ + HfO₂ 2.02-5 BaO  0.4-6 SrO   0-6MgO   0-2 CaO   0-2 Σ alkaline earth  0.4-6 metal oxides Li₂O  0.5-3,but not more than 25% of a sum total amount, in Mol % on said oxidebasis, of said alkali metal oxides. Σ SiO₂ + ZrO₂ + HfO₂ >58.5;

wherein a molar ratio of ZnO:Σ alkaline earth metal oxides >3.5:1; andwherein the cladding comprises a cladding glass and the cladding glasscontains, in Mol % on an oxide basis: SiO₂ 70-78 Al₂O₃  0-10 B₂O₃  5-14Li₂O 0 Na₂O  0-10 K₂O  0-10 MgO 0-1 CaO 0-2 SrO 0-1 BaO 0-3 F  0-1.


24. A step index fiber comprising core and a cladding, wherein the corecomprises a core glass and said core glass contains, in Mol % on anoxide basis: SiO₂ 54.5-65 ZnO 18.5-30 Σ alkali metal oxides   8-20 La₂O₃ 0.5-3 ZrO₂   2-5 HfO₂ 0.02-5 Σ ZrO₂ + HfO₂ 2.02-5 BaO  0.4-6 SrO   0-6MgO   0-2 CaO   0-2 Σ alkaline earth  0.4-6 metal oxides Li₂O  0.5-3,but not more than 25% of a sum total amount, in Mol % on said oxidebasis, of said alkali metal oxides. Σ SiO₂ + ZrO₂ + HfO₂ >58.5;

wherein a molar ratio of ZnO:Σ alkaline earth metal oxides >3.5:1, andwherein the cladding comprises a cladding glass and the cladding glasscontains, in Mol % on an oxide basis: SiO₂ 62-70 B₂O₃ >15 Li₂O >0.1 Na₂O 0-10 K₂O  0-10 MgO 0-5 CaO 0-5 SrO 0-5 BaO 0-5 ZnO 0-5 F  0-1.


25. A step index fiber comprising core and a cladding, wherein the corecomprises a core glass and said core glass contains, in Mol % on anoxide basis: SiO₂ 54.5-65 ZnO 18.5-30 Σ alkali metal oxides   8-20 La₂O₃ 0.5-3 ZrO₂   2-5 HfO₂ 0.02-5 Σ ZrO₂ + HfO₂ 2.02-5 BaO  0.4-6 SrO   0-6MgO   0-2 CaO   0-2 Σ alkaline earth  0.4-6 metal oxides Li₂O  0.5-3,but not more than 25% of a sum total amount, in Mol % on said oxidebasis, of said alkali metal oxides. Σ SiO₂ + ZrO₂ + HfO₂ >58.5;

wherein a molar ratio of ZnO:Σ alkaline earth metal oxides >3.5:1, andwherein the cladding comprises a cladding glass and the cladding glasscontains, in Mol % on an oxide basis: SiO₂ 75-85 Al₂O₃ 1-5 B₂O₃ 10-14Na₂O 2-8 K₂O  0-1.


26. A step index fiber comprising a core and a cladding, wherein thecore comprises a core glass and said core glass contains, in Mol % on anoxide basis: SiO₂ 55-61 ZnO 20-25 Σ alkali metal oxides 11-16 La₂O₃1.5-2.9 ZrO₂   2-4.08 HfO₂ 0.02-3.3  Σ ZrO₂ + HfO₂ 2.02-4.1  TiO₂  0-0.5BaO 0.4-5   SrO 0.4-5   Σ alkaline earth metal oxides 0.4-5   Li₂O 1-2.5Σ SiO₂ + ZrO₂ + HfO₂ >59;

wherein a molar ratio of ZnO:Σ alkaline earth metal oxides >3.5:1.
 27. Astep index fiber comprising core and a cladding, wherein the corecomprises a core glass and said core glass contains, in Mol % on anoxide basis: SiO₂   55-61 ZnO   20-25 Σ alkali metal oxides   11-16La₂O₃  1.5-2.9 ZrO₂   2-4.08 HfO₂ 0.02-3.3 Σ ZrO₂ + HfO₂ 2.02-4.1 BaO 0.4-5 SrO  0.4-5 Σ alkaline earth  0.4-5 metal oxides Li₂O   1-2.5 ΣSiO₂ + ZrO₂ + HfO₂ >59;

wherein a molar ratio of ZnO:Σ alkaline earth metal oxides >3.5:1 andwherein the cladding comprises a cladding glass and the cladding glasscontains, in Mol % on an oxide basis: SiO₂ 70-78 Al₂O₃  0-10 B₂O₃  5-14Li₂O 0 Na₂O  0-10 K₂O  0-10 MgO 0-1 CaO 0-2 SrO 0-1 BaO 0-3 F  0-1.


28. A step index fiber comprising a core and a cladding, wherein thecore comprises a core glass and said core glass contains, in Mol % on anoxide basis: SiO2   55-61 ZnO   20-25 Σ alkali metal oxides   11-16La₂O₃  1.5-2.9 ZrO₂   2-4.08 HfO₂ 0.02-3.3 Σ ZrO₂ + HfO₂ 2.02-4.1 BaO 0.4-5 SrO  0.4-5 Σ alkaline earth  0.4-5 metal oxides Li₂O   1-2.5 ΣSiO₂ + ZrO₂ + HfO₂ >59;

wherein a molar ratio of ZnO:Σ alkaline earth metal oxides >3.5:1, andwherein the cladding comprises a cladding glass and the cladding glasscontains, in Mol % on an oxide basis: SiO₂ 62-70 B₂O₃ >15 Li₂O >0.1 Na₂O 0-10 K₂O  0-10 MgO 0-5 CaO 0-5 SrO 0-5 BaO 0-5 ZnO 0-5 F  0-1.


29. A step index fiber comprising a core and a cladding, wherein thecore comprises a core glass and said core glass contains, in Mol % on anoxide basis: SiO₂   55-61 ZnO   20-25 Σ alkali metal oxides   11-16La₂O₃  1.5-2.9 ZrO₂   2-4.08 HfO₂ 0.02-3.3 Σ ZrO₂ + HfO₂ 2.02-4.1 BaO 0.4-5 SrO  0.4-5 Σ alkaline earth  0.4-5 metal oxides Li₂O   1-2.5 ΣSiO₂ + ZrO₂ + HfO₂ >59;

wherein a molar ratio of ZnO:Σ alkaline earth metal oxides 3.5:1; andwherein the cladding comprises a cladding glass and the cladding glasscontains, in Mol % on an oxide basis: SiO₂ 75-85 Al₂O₃ 1-5 B₂O₃ 10-14Na₂O 2-8 K₂O  0-1.